Supplement I: Greyhawk.
There is a chasm of difference between the original booklets and what appeared in Supplement I.
I think the scope of this change is often underestimated, but in terms of actually playing the game I'm struck by how similar my experience with OD&D is to my experience with BECMI, AD&D2, and 3E.
When I was young, I:
- Used AD&D1 supplements with BECMI and AD&D2.
- Freely intermixed BECMI and AD&D2 material without even thinking about it.
The editions of the game from '79 to '99, while certainly possessing some key distinctions, were inter-compatible to the point of "I don't need to even convert this stuff". So other than Basic's conflation of race and class and the change in XP methodology, it doesn't surprise me that no one is citing a significant rules-shift in this time period.
Now, for the DM's perspective, let me copy-paste from an e-mail exchange I had with my Dm who has:
- Used OD&D, AD&D, and BECMI modules with 3E.
- Used the same 3E modules in both 3E and OD&D.
- Used the same 4E modules in both 4E and OD&D.
(quote)
Unlike the BECMI/AD&D material, all of these obviously required mechanical conversion. But in each case I lazily followed the conversion process of least resistance: If the encounter says it gets 8 goblins, then I open up the local equivalent of the Monster Manual and use the stats for 8 goblins.
Here's what I experienced:
- The OD&D, AD&D, and BECMI modules all played fine in 3E.
- The 3E modules played pretty much identically in both 3E and OD&D.*
- The 4E module played radically differently in 4E and OD&D.
(end quote)
Speaking as a player, you can see a similar continuity in playing the core classes. From OD&D to 3E there was a gradual accumulation of new options for characters, but surprisingly little difference in how they played at a basic level: Fighters in OD&D play like fighters in AD&D, BECMI, and 3E; magic-users in OD&D play like wizards in AD&D, BECMI, and 3E; and so forth. But fighters and wizards in 4E don't play anything like their predecessors.
4E is the breakpoint at which the gameplay shifts on a fundamental level on both sides of the DM's screen. (Which is unsurprising, since the designers said they were doing that deliberately.)
On a non-mechanical level, you also have a basic continuity of implied cosmology from OD&D to 3E (with the same gradual accrual of additional bling). Here one finds a significant shift from AD&D1 to AD&D2, but 3E largely shifted back towards AD&D1 in this regard. But those shifts once again pale in comparison to the significant break we find in 4E from what came before.
I can understand people saying that 3E's process of cleaning up and unifying the system was a methodology that is shared by 4E's unified design scheme. But 3E created a unified system by polishing the gameplay of D&D. 4E created a unified system by jettisoning the gameplay of D&D.